/*

    @背景滚动

*/

var disRoll=0
function bgRoll(){
    disRoll+=5;
    if(disRoll>canvas.height){
        disRoll=0;
    }
    ctx.beginPath();
    ctx.drawImage(bgImg,0,disRoll,canvas.width,canvas.height);
    ctx.beginPath();
    ctx.drawImage(bgImg,0,disRoll-canvas.height,canvas.width,canvas.height);
    
}

